This invention relates to injection molding and more particularly to an improved heated nozzle and a method of manufacturing the same.
Injection molding nozzles or sprue bushings have a central bore for conveying pressurized melt towards a gate leading to a cavity, and it is well known in the art to use a helical electric heating element to provide heat to maintain the temperature of the melt within an acceptable range. One example of this is to locate the helical heating elements between inner and outer sleeves, but this has the disadvantage that the air gap between the sleeves in which the heating element is located acts as an insulator. More recently, this problem has been overcome by vacuum casting the heating element in a highly conductive material such as copper alloy, as shown in the applicant's U.S. Pat. Nos. 4,238,671 and 4,355,460 which issued Dec. 9, 1980 and Oct. 26, 1982 respectively. This same type of structure is shown in the applicant's U.S. Pat. No. 4,403,405 which issued Sept. 13, 1983 and relates to an improved connector assembly for the nozzle. While these nozzles with cast in heating elements have worked very well, they are relatively costly to manufacture.
It is, of course, well known to provide heat to a hollow sleeve or pipe by wrapping it with insulated heating wire or tape. However, this has not been found to be satisfactory for injection molding nozzles because it is inefficient and unreliable. With the increased use of more difficult to mold plastics materials, it has become necessary to maintain the melt within narrower and narrower temperature ranges. If it rises too high degradation of the melt will result, and if it drops too low, the melt will clog in the system and produce an unacceptable product, either of which can necessitate the system being shut down for a clean out. Furthermore, if heat is not quickly removed from the element, it can result in it overheating and burning out. This danger is, of course, greater at higher temperatures, but if heat is not quickly dissipated from the heating element, it must necessarily be operated at a higher temperature to provide sufficient heat. The higher temperature itself shortens the life of the heating element and this is worse if "hot spots" occur along the length of the wire. Therefore, wrapped heating elements have not worked because the heat is not removed quickly and uniformly enough for them to be to be efficient and reliable for the latest high temperature engineering materials.